1. Field of the Invention
The present invention concerns a method and a testing device for field quality testing of a magnetic resonance antenna arrangement of a magnetic resonance system, the magnetic resonance antenna arrangement being formed of multiple antenna elements. As used herein, “field quality testing” means testing a quality testing (in particular error checking) of the magnetic resonance antenna arrangement on location at the operating (installation) site, in particular in a situation located in the magnetic resonance measurement (data acquisition) chamber of the magnetic resonance system. Moreover, the invention concerns a magnetic resonance system with such a testing device and a magnetic resonance antenna arrangement.
2. Description of the Prior Art
Magnetic resonance tomography is a widespread method for acquisition of images of the inside of a body. In this method the body to be examined is exposed to a relatively high basic magnetic field, for example of 1.5 Tesla or even of 3 Tesla in newer systems (known as high magnetic field systems). A radio-frequency excitation signal (known as the B1 field) is then emitted, which causes the nuclear spins of specific atoms excited to resonance by this radio-frequency field to be tilted by a specific flip angle relative to the magnetic field lines of the basic magnetic field. The radio-frequency signal (known as the magnetic resonance signal) radiated upon relaxation (return to equilibrium) of the nuclear spins is then acquired with suitable antenna arrangements (called “magnetic resonance antenna arrangements” in the following). The raw data acquired in this manner are ultimately used to reconstruct the desired image data. Respective defined magnetic field gradients are superimposed on the basic magnetic field for spatial coding during the transmission and the readout (acquisition) of the radio-frequency signals.
The magnetic resonance antenna arrangement for acquisition of the magnetic resonance signals can be the same antenna arrangement that is used to emit the B1 field. Normally a so-called “whole-body coil” (also called “whole-body antenna” or “body coil”) is installed in the scanner unit to emit the B1 field in the scanner unit in which the magnetic resonance measurement chamber is located (usually in the form of a patient tunnel extending through the scanner unit). It is fashioned to emit a homogenous B1 field in an optimally large region inside the magnetic resonance measurement chamber. Typical antenna structures for such whole body coils are the known cage structure and the known saddle coil structure.
In the magnetic resonance examinations of specific sub-regions of a subject or a patient to be examined, arrangements known as “local coils” are increasingly used as antennas to acquire the magnetic resonance signals. In the examination, these local coils are arranged relatively close to the body surface directly at the examination subject of interest (for example a specific organ or body part). In contrast to the whole body coil, such local coils have the advantage of being able to be located closer to the regions of interest. The noise component caused by the electrical losses within the examination subject is thereby reduced so that the signal-to-noise ratio (SNR) of a local coil is in principle better than that of a farther-removed antenna. A single antenna element (for example in the form of a single conductor loop with a pre-amplifier) is, however, only able to generate an effective image within a defined spatial expanse that lies on the order of the diameter of the conductor loop. Therefore—and to minimize the measurement time with parallel imaging—most local coils are designed as multi-channel coils with a number of individual antenna elements, for example many individual conductor loops arranged in parallel like a matrix or overlapping one another, each normally connected to its own pre-amplifier. Presently local coils with up to 32 channels or individual antenna elements are already normally used. Local coils with up to 128 channels are in planning or in trial use. Such local coils can be mechanically designed in an arbitrary manner, for example as relatively flexible, flat antenna arrangements that are placed on, under or at the examination subject, or as stable cylindrical constructions for use as head coils or the like, for example.
In order to be able to ensure and check the functionality of a local coil even in the field (for example on location in the operating (installation) site), it is meaningful to create standardized and largely automated measurement methods that are automatically executed. With such a field quality check (generally also called a “QA check” (QA: quality assurance)), an operator of the magnetic resonance system can determine himself or herself (i.e. without the use of a service technician) whether a local coil is functioning properly or whether a defect exists at a single one of or multiple antenna elements. If such a QA check does not run successfully, the local coil is sent back to the manufacturer, who simultaneously receives information about the possible cause of error and the error location in the local coil as the result of the quality check. Presently such a field quality check is conducted with the tested coil being installed in a fixed, defined position in the scanner unit of the magnetic resonance system together with an imaging phantom. The field quality check is based on the generation of magnetic resonance images of the phantom that are automatically evaluated and tested for deviations from predefined thresholds. This method has several disadvantages.
Such quality tests last multiple minutes because complete magnetic resonance images must be generated. Furthermore, the use of a phantom is necessary for the testing, and this phantom must be positioned precisely according to a predetermined measurement specification. Errors in the positioning can lead to the situation of a local coil being incorrectly detected as defective. This generates high costs in returned goods traffic. Finally, such a quality test must always be conducted as an independent measurement that cannot simply be handled in a standard patient operation.